1. Field of the Invention
The present invention relates in general to the field of signal processing, and, more specifically, to apparatuses and methods for implementing discontinuous conduction mode (DCM) for a switch-mode converter and also to apparatuses and methods for implementing a switch-mode converter power factor corrector (PFC) which operates in both CCM and DCM.
2. Description of the Related Art
Power factor correctors often utilize a switch-mode boost stage to convert alternating current (AC) voltages (such as line/mains voltages) to direct current (DC) voltages or DC-to-DC wherein the input current is proportional to the input voltage. Power factor correctors provide power factor corrected (PFC) and regulated output voltages to many devices that utilize a regulated output voltage.
FIG. 1 represents a typical exemplary power factor corrector 100, which includes a switch-mode boost stage 102. Voltage source 101 supplies an alternating current (AC) input voltage Vin(t) to a full-wave diode bridge rectifier 103. The voltage source 101 (e.g., voltage Vin(t)) is, for example, a public utility, such as a 60 Hz/120 V line (mains) voltage in the United States of America or a 50 Hz/230 V line (mains) voltage in Europe. The input rate associated with input voltage Vin(t) is the frequency of voltage source 101 (e.g., 60 Hz in the U.S. and 50 Hz. in Europe). The rectifier 103 rectifies the input voltage Vin(t) and supplies a rectified, time-varying, line input voltage Vx(t) to the switch-mode boost stage 102. The actual voltage at any time t is referred to as the instantaneous input voltage. unless otherwise stated, the term “line rate” is hereafter referred to and defined as the rectified input frequency associated with the rectified line voltage Vx(t). The line rate is also equal to twice the input frequency associated with input voltage Vin(t). The rectified line input voltage is measured and provided in terms of Root Mean Square (RMS) voltage, e.g., Vrms.
The switch-mode boost stage 102 includes a switch 108 (e.g., Field Effect Transistor (FET)) by which it is controlled and provides power factor correction (PFC) in accordance with how switch 108 is controlled. The switch-mode boost stage 102 is also controlled by switch 108 and regulates the transfer of energy from the rectified line input voltage Vx(t) through inductor 110 to capacitor 106 via a diode 111. The inductor current iL ramps ‘up’ when the switch 108 conducts, i.e. is “ON”. The inductor current iL ramps down when switch 108 is nonconductive, i.e. is “OFF”, and supplies current iL to recharge capacitor 106. The time period during which inductor current iL ramps down is commonly referred to as the “inductor flyback time”. A sense resistor 109 is utilized effectively in series with inductor 110.
Power factor correction (PFC) controller 114 of power factor corrector 100 controls switch 108 and, thus, controls power factor correction and regulates output power of the switch-mode boost stage 102. The goal of power factor correction technology is to make the switch-mode boost stage 102 appear resistive to the voltage source 101. Thus, the PFC controller 114 attempts to control the inductor current iL so that the average inductor current iL is linearly and directly related to the rectified line input voltage Vx(t). Unitrode Products Datasheet entitled “UCC2817, UCC2818, UCC3817, UCC3818 BiCMOS Power Factor Preregulator” (SLUS3951) dated February 2000—Revised February 2006 by Texas Instruments Incorporated, Copyright 2006-2007 (referred to herein as “Unitrode datasheet”) and International Rectifier Datasheet entitled “Datasheet No. PD60230 rev C IR1150(S(PbF) and IR 11501(S)(PbF)” dated Feb. 5, 2007 by International Rectifier, describe examples of PFC controller 114. The PFC 114 supplies a pulse width modulated (PWM) control signal CS0 to control the conductivity of switch 108.
Two modes of switching stage operation exist: Discontinuous Conduction Mode (“DCM”) and Continuous Conduction Mode (“CCM”). In DCM, switch 108 of PFC controller 114 (or boost converter) is turned on (e.g., “ON”) when the inductor current iL equals zero. In CCM, switch 108 of PFC controller 114 (or boost converter) switches “ON” when the inductor current is non-zero, and the current in the energy transfer inductor 110 never reaches zero during the switching cycle. In CCM, the current swing is less than in DCM, which results in lower I2R power losses and lower ripple current for inductor current iL which results in lower inductor core losses. The lower voltage swing also reduces Electro Magnetic Interference (EMI), and a smaller input filter can then be used. Since switch 108 is turned “OFF” when the inductor current iL is not equal to zero, diode 111 needs to be very fast in terms of reverse recovery in order to minimize losses.
The switching rate for switch 108 is typically operated in the range of 20 kHz to 100 kHz. Slower switching frequencies are avoided in order to avoid the human audio frequency range as well as avoid increasing the size of inductor 110. Faster switching frequencies are typically avoided since they increase the switching losses and are more difficult to use in terms of meeting Radio Frequency Interference (RFI) standards.
Capacitor 106 supplies stored energy to load 112. The capacitor 106 is sufficiently large so as to maintain a substantially constant link output voltage Vc(t) through the cycle of the line rate. The link output voltage Vc(t) remains substantially constant during constant load conditions. However, as load conditions change, the link output voltage Vc(t) changes. The PFC controller 114 responds to the changes in link output voltage Vc(t) and adjusts the control signal CS0 to resume a substantially constant output voltage as quickly as possible. The PFC controller 114 includes a small capacitor 115 to prevent any high frequency switching signals from the line (mains) input voltage Vin(t).
PFC controller 114 receives two feedback signals, the rectified line input voltage Vx(t) and the link output voltage Vc(t), via a wide bandwidth current loop 116 and a slower voltage loop 118. The rectified line input voltage Vx(t) is sensed from node 120 between the diode rectifier 103 and inductor 110. The link output voltage Vc(t) is sensed from node 122 between diode 111 and load 112. The current loop 116 operates at a frequency fc that is sufficient to allow the PFC controller 114 to respond to changes in the rectified line input voltage Vx(t) and cause the inductor current iL to track the rectified line input voltage Vx(t) to provide power factor correction. The inductor current iL controlled by the current loop 116 has a control bandwidth of 5 kHz to 10 kHz. The voltage loop 118 operates at a much slower frequency control bandwidth of about 5 Hz to 20 Hz. By operating at 5 Hz to 20 Hz, the voltage loop 118 functions as a low pass filter to filter a harmonic ripple component of the link output voltage Vc(t).
A number of external components which are outside of PFC controller 114 for respective power factor corrector 100 are still required. For example, a power factor corrector 100 may typically comprise of two Proportional-Integral (PI) controllers, one PI controller associated with the current loop 116 and another PI controller associated with the voltage loop 118. It would be desired and would simplify the circuitry for the power factor corrector 100 if the PI controller associated with the current loop 116 could be eliminated. Furthermore, typical power factor corrector 100 does not primarily comprise of digital circuitry. Thus, it is needed and desired to have a power factor corrector which is primarily made up of digital circuitry. It is needed and desired to also minimize the number of external components outside of a PFC controller for a power factor corrector.
Furthermore, when the target current (e.g., average inductor current) for a switch mode converter (e.g., switch mode boost or buck converter/stage) is less than a minimum certain threshold, the switch mode converter can no longer operate in the continuous conduction mode (CCM) (e.g., inductor current iL falls below zero). Thus, a system and method for operating a switch mode converter in DCM is needed and desired. In particular, it is further needed and desired to provide a switch mode boost or buck converter Which can operate in both CCM and DCM for a Power Factor Corrector in which the design and calculations for implementing such operating modes is straight forward. Thus, it is needed and desired to have a switch-mode power factor corrector which is primarily made up of digital circuitry wherein the switch-mode stage of the PFC can operate in both CCM and DCM. It is needed and desired to also minimize the number of external components outside of a PFC controller for a power factor corrector which utilizes a switch-mode stage that can operate in both CCM and DCM.